feat: add build info (commit SHA + deploy timestamp) to health endpoint and admin sidebar

- deploy.sh writes .build-info with commit SHA and timestamp after git pull
- /health endpoint now returns version, commit, and deployed_at fields
- Admin sidebar footer shows version and commit SHA
- Hetzner docs updated: runner --config flag, swap, and runner timeout

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/deploy.sh

@@ -37,6 +37,13 @@ fi
 log "Restoring local changes …"
 git stash pop --quiet 2>/dev/null || true
 
+# ── 1b. Write build info ─────────────────────────────────────────────────────
+log "Writing build info …"
+printf '{"commit":"%s","deployed_at":"%s"}\n' \
+    "$(git rev-parse --short=8 HEAD)" \
+    "$(date -u +%Y-%m-%dT%H:%M:%SZ)" \
+    > .build-info
+
 # ── 2. Rebuild and restart containers ────────────────────────────────────────
 log "Rebuilding containers …"
 if ! $COMPOSE up -d --build; then